Reflective surface for solar energy collector

ABSTRACT

Concentrating solar collector systems that utilize a concentrating reflector to direct incident solar radiation to a solar receiver are described. In one aspect, the reflective surface is arranged to direct light to the receiver in a non-imaging manner in which the solar rays reflected from the opposing edges of the reflective surface are generally directed towards a central portion of the solar receiver. Rays reflected from selected central portions of the reflective surface are directed closer to the edges of the receiver than the solar rays reflected from the edges of the reflective surface. The described reflectors are generally intended for use in solar collector systems that track movements of the sun along at least one axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional PatentApplication No. 61/162,125 filed Mar. 20, 2009 which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The highest cost components of a solar photovoltaic system are the solarcells that convert sunlight to electricity by the photoelectric effect.To use these cells more effectively, concentrating photovoltaic systemsfocus sunlight from a larger aperture onto a smaller cell area. Althoughconcentrating photovoltaic designs use less active cell material, theytypically require additional structure such as mirrors, lenses and heatsinks, and are fundamentally limited to utilizing less then all of thetotal available light. These factors increase cost and system complexityand reduce the optical-to-electrical efficiency over non-concentratingphotovoltaic systems.

Although existing concentrating solar photovoltaic systems work well,there are continuing efforts to further improve the design and costeffectiveness of concentrating photovoltaic systems.

SUMMARY OF THE INVENTION

Concentrating solar collector systems that utilize a concentratingreflector to direct incident solar radiation to a solar receiver aredescribed. In one aspect, the surface of the reflector is arranged todirect light to the receiver in a non-imaging manner in which the solarrays reflected from the opposing edges of the reflective surface aregenerally directed towards a central portion of the solar receiver. Raysreflected from selected central portions of the reflective surface aredirected closer to the edges of the receiver than the solar raysreflected from the edges of the reflective surface. The describedreflectors are generally intended for use in solar collector systemsthat track movements of the sun along at least one axis.

A variety of reflector surface geometries are describe that facilitatethe described non-imaging reflection of the incident radiation. By wayof example, the reflector may include a plurality of reflectivesections, with at least some of the reflective sections having ageometry that varies from a reference parabola that approximates a crosssectional shape of the reflective surface. In various embodiments, oneor more sections of the reflector have curvatures that are greater thanthat of the reference parabola, while other sections have curvaturesthat are less than that of the reference parabola. By way of example, insome embodiments, the angular deviation of the reflective surface fromthe reference parabola varies substantially linearly such that a secondderivative deviation of the reflective surface from the referenceparabola is substantially constant. Each reflective surface section ispreferably angularly and spatially continuous. However, in someembodiments the reflector may be made up of more than one distinctreflector segments that are angularly and/or spatially discontinuousfrom each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1A is a diagrammatic cross-sectional view of a quarter parabolicreflector and a solar receiver as described in U.S. patent applicationSer. No. 12/100,726.

FIG. 1B is a simplified diagrammatic cross-sectional view of thereceiver of FIG. 1A.

FIG. 2A is a diagrammatic perspective view of a solar energy collectoraccording to one embodiment of the present invention.

FIG. 2B is a diagrammatic cross-sectional view of a reflective surfaceand a solar receiver according to one embodiment of the presentinvention.

FIG. 3A is a diagrammatic cross-sectional view of a reflective surfaceand a reference parabola according to one embodiment of the presentinvention.

FIG. 3B is an exemplary graph depicting the relative second derivativedeviation of a reflective surface from a reference parabola as afunction of relative X position according to one embodiment of thepresent invention.

FIG. 3C is an exemplary graph depicting the positional deviation of areflective surface from a reference parabola as a function of relative Xposition according to one embodiment of the present invention.

FIG. 3D is a diagrammatic cross-sectional view of a reflective surfaceand a receiver according to one embodiment of the present invention.

FIG. 3E is a diagrammatic plan view of a flux line on a solar panel anda receiver according to one embodiment of the present invention.

FIG. 3F is an exemplary graph depicting the position of a beam on areceiver as a function of the X position on a reflective surfaceaccording to one embodiment of the present invention.

FIG. 3G is an exemplary graph depicting intensity as a function of theposition on a receiver for sunlight reflecting from several reflectivesurface sections.

FIG. 3H is an exemplary graph depicting composite intensity as afunction of the position of a beam on a receiver.

FIG. 4 is a diagrammatic cross-sectional view of a sheet deformed bymandrels according to one embodiment of the present invention.

FIG. 5 is a diagrammatic cross-sectional view of a reflective surfaceand a receiver according to one embodiment of the present invention.

FIG. 6 is a diagrammatic cross-sectional view of a reflective surfaceand a receiver according to one embodiment of the present inventionwhere the reflective surface is near half parabolic section.

FIG. 7A is an exemplary graph depicting the relative angular deviationof a reflective surface from a reference parabola as a function ofrelative position along the reflective surface according to oneembodiment of the present invention.

FIG. 7B is an exemplary graph depicting the relative second derivativedeviation of the reflective surface of FIG. 7A from a reference parabolaas a function of relative position along the reflective surfaceaccording to one embodiment of the present invention.

FIG. 8A is a diagrammatic cross-sectional view of a receiver withadjacent secondary optics where the secondary optics are flat mirrors.

FIG. 8B is a diagrammatic cross-sectional view of a receiver withadjacent secondary optics where the secondary optics are outwardlycurved mirrors.

FIG. 9 is a diagrammatic cross-sectional view of a reflective surfaceand a receiver according to one embodiment of the present inventionwhere the reflective surface which concentrates sunlight on a singlereceiver is divided into a plurality of reflective surface sections.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic, not to scale and may notdepict intended curvatures and/or angles properly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates generally to concentrating photovoltaic(CPV) systems. Various aspects of the present invention relate to areflective surface that concentrates sunlight onto a solar receiver in alargely non-imaging manner. The reflective surface is arranged to helpminimize energy losses attributed to the misalignment of the edges ofthe reflective surface. Various embodiments of the reflective surfaceare angularly and spatially continuous and/or distribute light moreuniformly across the surface of the solar receiver. Such features canimprove the efficiency of solar energy collection and facilitate themanufacture of the reflective surface.

The present invention represents an improvement upon various reflectordesigns described in U.S. application Ser. No. patent application Ser.No. 12/100,726, entitled “Dual Trough Concentrating Solar PhotovoltaicModule,” filed Apr. 10, 2008, which is incorporated herein in itsentirety for all purposes and is referred to hereinafter as the '726application. To appreciate the advantages of the reflective surface, itis helpful to examine a parabolic reflector according to one embodimentof the invention described in the '726 application. FIG. 1A is across-sectional view of such a quarter parabolic reflector 102 and asolar receiver 106. Here the term quarter parabolic reflector refers toa parabolic segment used in a similar manner to that described in U.S.patent application Ser. No. 12/100,726, which is incorporated herein byreference. Quarter parabolic reflector 102, which extendslongitudinally, reflects incident sunlight 103 such that representativerays 108, which emanate from evenly spaced points on the parabolicreflector 102, would form a focal point 110. The focal point 110 issituated beyond the receiver 106, which causes rays 108 to be spreadacross the surface of the receiver 106 in a largely imaging manner FIG.1B is an enlarged view of the receiver 106, which has one or more solarcells 109.

The above approach, although effective in many applications, can beimproved. For example, the edges 101 of the parabolic reflector 102 aretypically more vulnerable to misalignment than the central portions ofthe parabolic reflector 102. Such misalignment can be caused by damage,wear and tear and/or tracking errors. In the illustrated embodiment,there is a fairly direct correspondence between the relative location onthe parabolic reflector 102 from which a ray 108 extends and therelative location on the solar cell 109 that the ray 108 contacts. Forexample, rays 108 that emanate from the edges 101 (such as rays 108 aand 108 b) and central portions (such as ray 108 c) of the parabolicreflector 102 tend to extend towards the edges and central portions,respectively, of the solar cell 109. If the aforementioned misalignmentalters the trajectory of rays emanating from the edges 101 of theparabolic reflector 102 (e.g., rays 108 a and 108 b), the rays may missthe solar cell 109 entirely.

It should also be appreciated that the rays 108 are not evenly spreadacross the solar cell 109. In the illustrated embodiment, the number ofrays 108 in the upper half of solar cell 109 is greater than the numberof rays 108 in the lower half. This indicates that the light intensityon some portions of the solar cell 109 is significantly greater than inother portions. Such an uneven distribution can result in the formationof regions of particularly high current density on the surface of thesolar cell 109, which in turn can lead to the formation of hightemperature “hot spots” that degrade the performance, reliability andefficiency of the solar cell 109.

Various embodiments of the present invention pertain to a solar energycollector with a reflective surface configured to address at least someof the above concerns. In the described embodiments, light raysreflected from lower tolerance regions of the reflective surface may bedirected to regions of the receiver that can better accommodateunintended changes in the trajectory of the light rays. For example, thereflective surface directs light from its edges towards the centralportions, rather than the edges, of a solar cell, receiver and/or fluxline. Various embodiments concentrate light in a largely non-imagingmanner and distribute it more uniformly across the surface of a solarcell.

FIG. 2A is a perspective view of a solar energy collector 200 accordingto one embodiment of the present invention. The collector 200, which hasa dual trough design, includes a support structure 202 that is arrangedto support a reflector structure 207. Application of this invention isnot limited to dual trough collector designs, but the invention may bebeneficially applied to a wide range of trough style collector designs.The reflector structure 207 has multiple reflector panels 206 with oneor more reflective surfaces. The reflective surfaces of the reflectorpanels 206 are arranged to direct incident sunlight toward solarreceivers 204, which each include one or more solar cells and arecoupled near the top edges of reflector structure 207. The collector 200can include any other feature described in the '726 application as well.

FIG. 2B is an enlarged, cross-sectional view of a reflective surface 250of the reflector panel 206 and a solar receiver 204. As indicated byrays 256, incident sunlight 252 is reflected off of reflective surface250 to form a flux line 260 on the receiver 204. (The flux line 260 canbe understood as an illuminated region on the receiver that is formed atleast substantially from the incident sunlight reflected from thereflective surface 250.) A reflective surface of this type has beenreferred to as a quarter parabola segment in the '726 application.

The design of the reflective surface 250 helps maximize the reception ofsolar energy by receiver 204 and keep the flux line 260 within theboundaries of the solar cells (not shown) on the receiver 204. To thisend, the edges 254 of the reflective surface 250, which can be prone tomisalignment due to manufacturing and/or tracking errors, are configuredto reflect rays towards a central region of the flux line 260 and/or thereceiver 204. In the illustrated embodiment, rays that are reflectedcloser to the outer edges of reflective surface 250 (e.g., rays 256 aand 256 b from edges 254 a and 254 b) tend to be directed more towardsthe center of receiver 204. Rays that are reflected closer to centralportions of the reflective surface 250 (e.g., rays 256 c and 256 d fromcentral portions 255 a and 255 b) tend to be directed more towards theedges of receiver 204 than the rays reflected closer to the edges of thereflective surface 250. This configuration can reduce solar energylosses by helping to direct rays from portions of the reflective surface250 (e.g., edges 254) that have lower tolerance to portions of thereceiver 204 that can tolerate misaligned rays with minimal impact oncell performance (e.g., central portions of the receiver 204 and/or fluxline 260).

The receiver 204 and the reflective surface 250 can be arranged invarious ways, depending on the needs of a particular application. Invarious embodiments, the receiver 204 and reflective surface 250 arepositioned such that at least rays 256 a and 256 b, which emanate fromthe outer edges of reflective surface 250, intercept each otherapproximately in or near the center of the flux line 260. A trackingsystem can help position the reflective surface 250 such that incomingsunlight 252 is substantially normal to the directrix (not shown) of areference parabola 262 upon which reflective surface 250 is based.

The reflective surface 250, unlike reference parabola 262, does not forma parabolic curve with a single directrix and focus. In variousembodiments, each one of the sections 258 of the reflective surface 250may form a different parabolic curve with a distinct directrix andfocus. As a result, the reflective surface 250 does not produce a singlefocus and concentrates light in a substantially non-imaging manner.

In some embodiments, there is a relationship between points on thereflective surface 250 and points on the reference parabola 262. Forexample, various points on the reflective surface 250 can deviate by apredetermined amount from the corresponding points on the referenceparabola 262. The amount and type of deviation can depend on the section258. (In the illustrated embodiment, three sections 258 a, 258 b and 258c are described, although there could be fewer or more sections.) Forexample, there is a higher curvature at various points in sections 258 aand 258 c than in the corresponding points of the reference parabola262. There is a lower curvature at various points in section 258 b thanin the corresponding points of the reference parabola 262. The amount ofcurvature deviation from the reference parabola can be substantiallyconstant, the same and/or different for various points within any givensection. The shape of the edges 254 of the reflective surface 250 can besubstantially identical to the shape of the edges of the referenceparabola 262. That is, at least portions of the edges 254, which overlapthe edges of the reference parabola 262, may reflect light in the sameway and have the same spatial orientation and position as the overlappedportions of the reference parabola 262.

Another embodiment of a reflective surface 300, its correspondingsections 312 and a reference parabola 302 will be described withreference to FIG. 3A. In the illustrated embodiment, the reflectivesurface 300 is substantially symmetrical (e.g., section 312 c interceptsas much incoming sunlight as section 312 a), although other relationsbetween the section sizes are also possible. The shape of each section312 of the reflective surface 300 is partly based on correspondingsections of the reference parabola 302.

Reference parabola 302 is at least partially defined by a directrix (notshown), which is parallel to the X-axis, 310, and perpendicular to they-axis 311. The general form for the mathematical equation thatdescribes a parabola is (Ax+By)²+Cx+Dy+E=0, where A, B, C, D, and E areconstants. Equivalently a parabola is defined as the loci of pointsequidistant from a point, called the focus, and a line, called thedirectrix. The reference parabola 262 has a directrix parallel to theX-axis, allowing the equation defining the reference parabola to besimplified to y=k*(x−a)²+b, Equation (1), where a, b, and k areconstants. Since the reference parabola has an upward facing opening kis positive. Taking the second derivative of the preceding parabolafunction yields d²y/dx²=k/2, implying that the second derivative isconstant independent of position. The reference parabola curvature, κ,is defined as κ=(d²y/dx²)/([1+(dy/dx)²]̂3/2) or equivalentlyκ=k/([1+(dy/dx)²]̂3/2). The curvature is thus proportional to k and thesecond derivative. The shape of each section 312 of the reflectivesurface 300 may be defined, at least partially, by a variation in k orcurvature from the reference parabola 302.

The form of the reference parabola 302 can vary widely. Although onlyone shape for the reference parabola 302 is shown, any of an almostinfinite range of parabolic shapes can be used, depending on the needsof a particular application. The term “parabola” can be defined invarious ways known in the art. In the illustrated embodiment, thereference parabola 302 is represented by a curve that begins and ends atthe same endpoints as the curve formed by the cross-sectional view ofthe reflective surface 300 and is made of a locus of points that areequidistant from a focal point and a directrix. The reference parabola302 can help determine various parameters of the reflective surface 300.For example, portions of the edges 306 of the reflective surface 300 andportions of the edges of the reference parabola 302 can overlap and havethe same position and slope. Points along the curve formed by reflectivesurface 300 can be designed in part based on corresponding points on thereference parabola 302.

Various approaches can be taken in corresponding points between thereference parabola 302 and the reflective surface 300. In theillustrated embodiment, the point-to-point correspondence is based onX-axis 310. For example, point 308 a on reflective surface 300corresponds with point 308 b on reference parabola 302, because theyshare the same relative X position on X-axis 310. Such correlations canbe based on other and/or additional metrics, such as a differentlyconfigured axis, an equation relating distances along the referenceparabola 302 and the reflective surface 300, etc.

In the illustrated embodiment, the second derivative of a particularpoint on reflective surface 300 is based at least partly on the secondderivative of a corresponding point on the reference parabola 302 and anadditional second derivative value that may vary depending on thesection 312 that the point is situated in. The additional secondderivative value can be at least substantially the same and/or differentfor two or more sections 312.

FIG. 3B depicts the second derivative deviation of the reflectivesurface 300 relative to the reference parabola 302 as a function of therelative X position along X-axis 310 of FIG. 3A. In the illustratedembodiment, the amount of second derivative deviation tends to besubstantially constant within a section 312, although this is not arequirement. For example, for most points in sections 312 a and 312 c ofthe reflective surface 300, the second derivative at a particular pointis equal to the second derivative at the corresponding point on thereference parabola plus a second derivative value Y, Y being a positivevalue. For most points in section 312 b, the second derivative at aparticular point is equal to the second derivative at the correspondingpoint on the reference parabola plus a second derivative value X. Inthis example, X is equal to −Y. The absolute value of the additionalsecond derivative value does not have to be the same for two or moresections 312, although such a feature can help simplify the manufactureand design of the reflective surface 300. It should be noted that thereare transitional regions 322 in which the relative second derivativedeviation changes from positive Y to X. Such regions may be situatedbetween sections 312 or constitute parts of sections 312. The rate ofsuch change is depicted as having a constant slope, although the rate ofchange could be increasing or decreasing.

FIG. 3C illustrates the positional deviation of the reflective surface300 from the reference parabola 302 as a function of the relative Xposition using the X-axis of FIG. 3A. Dashed lines indicate the parts ofthe graph 390 that correspond to sections 312 a, 312 b and 312 c of FIG.3A. In section 312 a, the positive second derivative deviation (as shownin FIG. 3B) causes the reflective surface 300 to bend away from thereference parabola 302 at an increasing rate. In section 312 b, thenegative second derivative deviation causes the reflective surface 300to bend back toward the reference parabola 302. In section 312 c, thepositive second derivative deviation causes the reflective surface 300to bend toward the reference parabola 302 at a decreasing rate.

The smooth profile of the curve in graph 390 indicates that thereflective surface 300 is spatially continuous. Reflective surface 300can also substantially lack any angular discontinuities and/or sharpedges. In some embodiments, a function approximating the reflectivesurface 300 and the first derivative of the function are continuousacross the entire reflective surface 300. Mathematically one method toaccomplish this is by adjusting the values of the constants a and b inEquation 1 so that functions which describe the various sections 312have equal values and first derivatives at their boundaries. In thiscase the directrices that partially define all of the parabolic sectionsare parallel. Spatial and angular continuity can be advantageous for atleast two reasons. First, spatial and/or angular discontinuities cancreate difficulties in manufacturing. Sheets, for example, that includesuch discontinuities can be more prone to breaking and can require moretooling to produce.

The reflective surface 300, for example, can be formed from a singlesheet of reflective material and need not be formed from separate sheetsor pieces that have been welded, adhered and/or bonded together. Second,sharp edges can promote the scattering of incoming sunlight. This canmake it more difficult to maximize the concentration of sunlight on areceiver.

Referring to FIGS. 3D and 3E, various approaches for arranging thereflective surface 300, the incident light 330 and the solar receiver340 will be described. Reflective surface 300 reflects incident sunlight330 and directs rays 332 toward the solar receiver 340. The incomingsunlight 330 is substantially normal to the directrix (not shown) of thereference parabola 302. The sunlight reflected by the reflective surface300 forms a flux line 334 (with a width 338) on the receiver 340.

In various embodiments one or more sections 312 of the reflectivesurface 300 can form a distinct parabolic curve with a distinctdirectrix and focus. The foci corresponding to the different sections312 may not be coincident and the directrices corresponding to thedifferent sections 312 may not be parallel. As a result, incidentsunlight 330, which is preferably substantially normal to the directrixof the reference parabola 302, is concentrated in a non-imaging manner.The edges 306 of the reflective surface 300 may have substantially thesame spatial orientation and slope as at least portions of the edges ofthe reference parabola 302. Such edge portions thus reflect light in amanner similar to the reference parabola 302. In the illustratedembodiment, for example, the outer edges of the reflective surface 300direct light rays 332 a and 332 b towards a point 335. Edges 306 of thereflective surface 300 are arranged and distanced from the receiver 340such that the point 335 is in a central region of the flux line 334and/or the receiver 340. This approach is different from the onedescribed in connection with FIG. 1B, where the focal point 210 wassituated behind the receiver 206. Such an arrangement and distancinghelps reflective surface 300 form a more uniform flux line 334 and/ordirect light more accurately from the edges 306 of the reflectivesurface 300 to the more central portions of the receiver 340.

FIG. 3E provides an enlarged plan view of the receiver 340 and the fluxline 334, which is marked by the shaded region. In FIG. 3E, the view ofthe receiver 340 has been rotated 90° relative to the cross-sectionalview provided in FIG. 3D. The receiver 340 may have one or more solarcells 343 that can extend longitudinally across the receiver 340. Thesolar cells 343 use the concentrated incident solar energy to directlyproduce electricity. Alternatively, the receiver may use theconcentrated incident solar energy to heat a fluid or perform some otherbeneficial operation. The flux line 334 can be understood as a region onthe solar receiver 340 that is illuminated through sunlight reflectedfrom the reflective surface 300 of FIG. 3D. In some embodiments, thecenter of the flux line 345 is substantially nominally coincident withthe center of the receiver 340 and/or solar cells 343. The flux line 334can extend over all, substantially all and/or the majority of thesurface area of the one or more solar cells 343. Alternatively the fluxline 334 may extend over a minority of the solar cell surface area 343,which provides tolerance for tracking errors and mechanical inaccuraciesin the collector assembly. FIG. 3E depicts the flux line 334 asintersecting with a portion of the receiver 340 that is not part of thesolar cell 343 i.e., the region near the beveled edge of the cell 343,although in various embodiments the flux line 343 may be entirely withinthe periphery of the one or more solar cells 343. In the illustratedembodiment, the flux line 334 does not extend over buffer regions 344 aand 344 b of the solar cell 343 and the width of the flux line 338 isslightly smaller than the width 339 of the solar cell 343 and the widthof the receiver 337. Buffer regions 344 can be located anywhere alongthe periphery of the cell 343, such as along the top and bottom edges ofthe cell 343. Although reflective surface 300 is designed to targetreflected rays within the boundaries of flux line 334, manufacturingand/or tracking errors can cause reflected light to strike the receiver340 in a region outside of the intended flux line 334. Buffer regions344 can catch errant rays and help reduce the loss of solar energy.

FIG. 3F is a graph 350 that maps various points on reflective surface300 to points on the receiver 340 of FIG. 3D. The vertical axisrepresents positions along the width of the receiver 337 of FIG. 3E. Thedistance between y1 and y3 on the vertical axis represents the width ofthe flux line 338 of FIG. 3E and the value of 0 along the vertical axisindicates the center 345 of the width of the flux line 338. Thehorizontal axis represents a relative X position on the reflectivesurface 300 (e.g., based on X axis 310 of FIG. 3A.) Dotted lines 314delineate sections 312 a, 312 b and 312 c. The graph 350 indicates wherea particular ray that is reflected off of a point on the reflectivesurface 300 intersects with receiver 340. For example, point 352indicates that incident sunlight reflects off point x2 in section 312 aof the reflective surface 300 and is directed toward point y2 in thelower half of the flux line 334.

The graph 350 illustrates how the various sections 312 can help todistribute light more evenly across flux line 334. In the illustratedembodiment, section 312 a directs light toward the lower half of theflux line 334 (e.g., the shaded region below center 345 in FIG. 3E)Section 312 c directs light toward the upper half of the flux line 334(e.g., the shaded region above center 345 in FIG. 3E.) The middlesection 312 b directs light over substantially the entire width of theflux line 338. Generally speaking, in the graph 350 two points (e.g., x2and x4) on the reflective surface 300 map to one point on the receiver340 (e.g., y2.). This can be untrue for parts of the curve of graph 350that correspond to the center and edges of the flux line 334. In theillustrated embodiment, for example, the center of the flux line 345(i.e., the value 0 on the vertical axis) corresponds to at least threepoints (e.g., x1, x5 and x7) on the reflective surface 300. The extremeouter edges of the flux line 334, which are positioned at y1 and y3,each correspond to only one point on the reflective surface 300 (i.e.,points x3 and x6 respectively.)

It should also be appreciated that the edges of reflective surface 300,which correspond to x1 and x7 on horizontal axis of graph 350, map tothe center of the flux line 345, which is designated with a 0 on thevertical axis of graph 350. Generally, the closer a point on thereflective surface 300 is to x1 or x7, the more the corresponding raysare directed towards the center of the flux line 345 and/or receiver 340(i.e., the value of 0 on the vertical axis.) The edges of a reflectivesurface can be particularly vulnerable to damage, manufacturing defectsand/or other sources of misalignment. If the edges of the reflectivesurface 300 are designed to direct light rays toward the outer edges ofthe flux line (as is the case with rays emanating from the edges of thereflective surface 102 of FIG. 1A), such misalignment can cause thelight rays to fall outside of the intended flux line and perhapsentirely miss the solar cell on the receiver, which results in a loss ofsolar energy. Directing rays from the edges of the reflective surface300 toward the center of the flux line 345 and/or receiver 340 canreduce the likelihood of such losses.

FIG. 3G includes a graph 370 that indicates light intensity as afunction of a position along the width of the flux line 338 of FIG. 3E.The value of 0 on the horizontal axis corresponds to the center of theflux line (e.g., center 345 of FIG. 3E). Curves 372 a, 372 b and 372 ccorrespond to sections 312 a, 312 b and 312 c respectively of thereflective surface 300 of FIG. 3A. The graph 370 indicates that section312 a (as represented by curve 372 a) directs light primarily over thelower half of the width of the flux line 338 of FIG. 3E. Middle section312 b (as represented by curve 372 b) directs light fairly uniformlyover the entire width of the flux line 338. Section 312 c (asrepresented by 372 c) directs light primarily over the upper half of thewidth of the flux line 338.

FIG. 3H includes a graph 380 showing the relative contributions of lightreflected from sections 312 a, 312 b and 312 c to the compositeintensity at various points along the width 338 of the flux line 334.This graph 380 indicates that the overall intensity across the width 338of the flux line 334 is relatively stable and uniform. In someembodiments, the variation in intensity in a central region 375 of theflux line 334 is approximately +1-10% or less. In another embodimentsuch variation is approximately +/−20% or less. The central region 375can be defined as a portion of the flux line 334 that accounts forapproximately 90% of the energy of the flux line 334, although otherdefinitions are also possible. A more uniform light intensity can helpimprove heat dissipation from the solar cells, avoid resistive lossesand promote cell efficiency.

In another aspect of the present invention, various exemplary methodsfor forming reflective surfaces are described. FIG. 4 presents oneapproach for manufacturing a reflective surface such as the reflectivesurface 300 of FIG. 3A. A substantially flat sheet of reflectivematerial is positioned between the mandrels 402 in FIG. 4. The mandrels402 each have a radius R, although two or more of the mandrels can havedifferent radii as well. The flat sheet is inelastically deformed by themandrels 402. The resulting deformed sheet 404 can be understood ashaving three sections 406 a, 406 b and 406 c. (Once the processing ofthe deformed sheet 404 is completed, these sections 406 a, 406 b and 406c can correspond to and have the features of sections 312 a, 312 b and312 c respectively of FIG. 3A.)

In the illustrated embodiment, each of the three sections 406 has acurvature of radius R. Two or more of the sections 406 could havedifferent curvatures that correspond to the radius of their underlyingmandrels 402. The centers of curvature 408 a and 408 c for sections 406a and 406 c, respectively, are situated below and face a first surfaceof the deformed sheet 404. The center of curvature 408 b for section 406b is situated over and faces a second surface of the deformed sheet 404that opposes the first surface. In some embodiments, inflection pointssuch as inflection points 410 a and 410 b divide the sheet into portionsthat curve in a first direction or in an opposite second direction. Thearea of the portions that curve in the first direction can beapproximately equal to the area of the portions that curve in the seconddirection. After the deformed sheet 404 has been shaped, it can be bowedto form a reflective surface having any of the features of reflectivesurface 300 of FIG. 3A. The bowing force can be applied at least in partby securing the deformed sheet 404 to a plurality of shaping ribs, as isdiscussed in previously cited '726 application.

Another approach to manufacturing reflective surfaces is described inFIG. 5. FIG. 5 illustrates a cross-sectional view of a reflectivesurface 500 and solar receiver 508. Reflective surface 500 has regions502 a and 502 b, midpoint 510 and outer edges 512 a and 512 b. Nominallyincident sunlight 504 is reflected as rays 506 to form flux line 511 onthe receiver 508.

The spatial and/or angular orientation of the reflective surface 500 canbe configured such that various portions of the reflective surface 500direct light to desirable sites on the receiver 508. In the illustratedembodiment, the outer edges 512 a and 512 b and the midpoint 510 of thereflective surface 300 are configured to direct corresponding rays 506a, 506 b and 506 c to the approximate center of the flux line 511. Thatis, particular portions of the receiver 508, such as the center of theflux line 511, are better positioned to capture solar energy even fromrays that deviate slightly from their intended path. A ray, for example,that is originally directed at the center of the flux line 511 but thatstrays slightly from the exact center will likely still be received by asolar cell and usefully converted into energy. This may be less true fora ray that is targeted at an edge of a solar cell and/or a flux line. Ifsuch a ray deviates from its intended course, it may have a higherlikelihood of intersecting with the receiver at a point outside theperiphery of the solar cell. As a result, reflective surface 500 isconfigured such that rays reflected from lower tolerance regions of thereflective surface 500, such as outer edges 512 a and 512 b, aredirected toward regions of the receiver 508 that can better accommodatesuch changes in the rays' trajectory, such as the center of the fluxline 511. In various embodiments, region 502 a, which is situatedbetween upper edge 512 a and the midpoint 510, can be configured todirect rays 506 across the lower half 511 b of the flux line, such thatthe lower half 511 a is substantially uniformly illuminated, althoughthis is not a requirement. Region 502 b, which is situated between themidpoint 510 and the lower edge 512 b, can be optionally configured tosubstantially uniformly illuminate at least a portion of the upper half511 a of the flux line. A function defining the reflective surface 500can be developed such that the function and its first derivative arecontinuous across the entire reflective surface 500 and its secondderivative is continuous across each region 502. The reflective surface500 and the receiver 508 can be further arranged to incorporate any ofthe features discussed in connection with the other figures in thisapplication.

The embodiments discussed above with respect to FIGS. 2 and 5 involvereflector segments that generally corresponded to quarter parabolicsegments. Such arrangements are well suited for use in applications suchas full trough collectors in which the receiver is located relativelyfar from the reflective surface, as for example, adjacent an upper edgeof an opposing reflector. It should be apparent that the same principlescan readily be applied to concentrating reflectors having a wide varietyof geometries and regardless of where the receiver sits relative to thereflector surface. For example, in many applications it may be desirableto utilize a parabolic segment that has a shorter focal length than thereflector segment illustrated in FIG. 5. One such embodiment isillustrated in FIG. 6.

In the embodiment of FIG. 6, a reflector 650 includes upper reflectoredge 654 a and lower reflector edge 654 b. The reflector 650 has a shapethat varies from a reference parabola segment 662 that has a focal pointcloser to the reflector than the embodiments illustrated in FIGS. 2 and5. The reference parabola 662 is sometimes referred to herein as a nearhalf parabola segment because lower edge 654 b of the reflector is nearthe parabola's axis of symmetry. Stated another way, the bottom portionof the reference parabola is nearly parallel with the referenceparabola's directrix.

The use of half or nearly half parabola reflectors has some potentialadvantages. Initially, the average distance between the reflectivesurface 650 and the flux line 660 is significantly less than the averagedistance in the quarter parabola segment illustrated in FIG. 2B. Thecloser flux line reduces the sensitivity of the system to trackingerrors and mechanical imperfections since angular errors in thereflection result in less displacement at the flux line. In general, areflector having a half parabola shape may have up to twice the apertureas a quarter parabola reflector for a given maximum distance between thereflector and the receiver.

Generally, rays of incoming sunlight 652 strike the reflective surface650 at varying angles of incidence, Θ. The angle between the incidentand the reflected rays is twice the angle of incidence, i.e., 2Θ. Raysincident at the lower trough edge 654 b have the smallest angle ofincidence, Θ_(min). Rays incident at the upper trough edge 654 a havethe largest angle of incidence, Θ_(max). The minimum and maximum anglesof incidence will vary with the collector design although in nearly halfparabola designs, Θ_(min) tends to be in the range of 0° toapproximately 20° degrees and Θ_(max) tends to be in the range of 35° to55°.

The receiver 604 may be oriented to minimize the angle of incidence ofthe reflected sunlight on the receiver. Minimizing the angle ofincidence on the receiver has the advantages of reducing reflectivelosses on the receiver's optical surfaces because reflective losses tendto increase at larger angles of incidence. It also has some effect onminimizing the size of the flux line since the receiver is oriented asperpendicular as possible to the reflected sunlight. The optimalorientation for the receiver will vary based on the nature of thereflector design. In one specific example, if the range of incidenceangles for a particular reflector have a range of 10° to 50° relative toa horizontal axis, it may be desirable to angle the receiver face at anangle on the order of 30° to minimize reflective losses. It should beappreciated, however, that although such an alignment of the receiverface may help reduce reflective losses, other factors such as thermalconsiderations may influence the actual receiver face orientation for aparticular reflector design. Although the receiver orientationoptimization has been described in the context of a near half parabolacollector design, it should be appreciated that the orientation of thereceiver may be optimized in this manner in any of the concentratingcollector designs including the embodiments of FIGS. 2 and 5.

As with the previous described embodiments, the upper and lower edges654 a, 654 b of the reflector 650 may be arranged to direct incidentlight towards the center of the receiver flux line. The reflector may bedivided into three segments as discussed with respect to FIG. 5, or inany of a variety of other manners. By way of example, an alternativereflector geometry that is designed to provide a small, well-defined,uniform intensity, flux line that avoids a high intensity focal spot infront of the receiver will be described in conjunction with FIGS. 7A and7B. A number of points on the lateral cross-section of the reflectorsurface shown in FIG. 6 are worthy of mention in this describedembodiment. These include the upper edge 654 a, the lower edge 654 b,the midpoint 654 c, and a number of inflection points 655 a-j. Forclarity is noted that the midpoint 654 c is the midpoint of thecollector aperture. That is, the midpoint 654 c is a midpoint of thereflective surface relative to an X-axis that is parallel to thedirectrix of the reference parabola 662. Thus, it should be appreciatedthat the midpoint 654 c is not positioned halfway between the upper edge654(a) and the lower edge 655(b) of the reflector as viewed along thereflective surface. Similarly, it should be appreciated that in thediscussion of the inflection points 655 a-j below, the inflection pointsrefer to positions of the reflective surface as defined relative to theX-axis.

FIGS. 7A and 7B b illustrate the angular and curvature deviation from areference parabola associated with a more complex reflector geometry.FIG. 7A illustrates the relative angular deviation of the reflectorsurface relative to the reference parabola. FIG. 7B illustrates thesecond derivative deviation relative to the reference parabola. Theangular deviation (a) of the reflective surface from the referenceparabola may be defined mathematically using the following formula:

α=Arctangent(dy/dx)

where dy/dx is the first derivative of the mathematical function thatdefines the reflective surface. FIG. 7B shows the corresponding secondderivative deviation of the reflective surface relative to the referenceparabola.

As best illustrated in FIG. 7A, starting at the upper edge 654 a of thereflector surface, the reflective surface angle equals that of thereference parabola. The receiver is positioned such that reflected raysfrom this position strike the center of the receiver. The angulardeviation from the reference parabola then increases linearly (by anegative amount) to point 655 a on the reflector surface. The linearityof the angular deviation is reflected by a constant negative secondderivative in the corresponding graph of FIG. 7B. It is noted thatmathematically, the fact that the second derivative deviation from areference parabola is constant and non-zero suggests that the curvatureof the reflector surface in such a region is also parabolic in shape,although the focal point of such a parabolic segment will be displacedrelative to the focal point of the reference parabola.

From point 655 a to 655 b, the changes in the curvature of the reflectorsurface match the changes in the curvature of the reference parabola ascan be seen in both FIGS. 7B, which means that the angular deviationfrom the reference parabola remains nominally constant in this region asseen in FIG. 7A. From point 655 b to 655 c, the angular deviation fromthe reference parabola is reduced linearly such that at point 655 c,angular deviation of the reflector surface from the reference parabolais less than it was at point 655 b. From point 655 c to 655 d, changesin the curvature of the reflector surface match the changes in thecurvature of the reference parabola such that the angular deviation fromthe reference parabola remains nominally constant in this region. Frompoint 655 d the angular deviation from the reference parabola is furtherreduced linearly until point 655 e at which point the reflector surfacehas an angle that is coincident with the angle of the referenceparabola.

From point 655 e to 655 f, the curvature of the reflector surfacematches the curvature of the reference parabola such that the reflectorsurface curvature matches the curvature of the reference parabola inthis region (although the reflector surface would be physically locatedbehind the reference parabola). From point 655 f, the angular deviationfrom the reference parabola is further increased linearly to point 655 gon the reflector surface such that the reflector surface angle nowdeviates from the reference parabola by a positive amount. From point655 f to 655 h, changes in the curvature of the reflector surface matchthe changes in the curvature of the reference parabola such that theangular deviation from the reference parabola remains nominally constantin this region (and thus the second derivative in this region is againzero). From point 655 h, the angular deviation from the referenceparabola is further increased linearly until point 655 i which resultsin an even greater angular deviation from the reference parabola. Inthis region the second derivative has a constant value.

From point 655 i to 655 j, changes in the curvature of the reflectorsurface again match the changes in the curvature of the referenceparabola such that the angular deviation from the reference parabolaremains nominally constant in this region. The angular deviation fromthe reference parabola is then decreased linearly from point 655 j tothe lower edge 654 b of the collector.

In general, regions of negative linear angular deviation correspond toconstant negative second derivative deviation values. These regionsfocus light behind the reference parabola focal point. The regions ofconstant angular deviation correspond to values of zero secondderivative deviation. These regions effectively focus light in a mannersimilar to the reference parabola. The regions of positive linearangular deviation correspond to constant positive second derivativedeviation values. These regions focus light in front of the referenceparabola focal point.

The reflective surface profile depicted in FIGS. 7A and 7B has thedesirable attribute of producing a small, well-defined,uniform-intensity flux line. It also has the advantage of avoiding ahigh intensity focal spot in front of the receiver. As best seen in FIG.7B, the regions at the edges of the reflective surface have smallersecond derivative values than the reference parabola (i.e., the region654 a-655 a and the region 655 j-654 b). These regions have lesscurvature than the reference parabola and produce foci behind thereceiver. The receiver therefore blocks rays reflected from thesesegments before they reach the foci.

The central region of the reflector is divided into eight smallersegments. Four of these segments have positive second derivativedeviation values (i.e., regions 655 b-655 c; 655 d-655 e; 655 f-655 gand 655 h-655 i). These four regions have an increased curvaturerelative to the reference parabola and therefore each have a focus infront of the receiver. However, since the reflective area associatedwith each positive curvature deviation region is relatively small, theintensity of the associated focus is more modest. The other foursegments in the central region have a second derivative deviation valueof zero, which means that their rate of angular change matches that ofthe reference parabola (i.e., regions 655 c-655 d; 655 e-655 f; 655g-655 h and 655 i-655 j). Thus, they focus in a manner similar to thereference parabola.

In the illustrated embodiment, the absolute values of each of thenon-zero second derivative deviations from the reference parabola arethe same. Reflector segments that have a zero second derivativedeviation from the reference parabola are interspersed between adjacentnon-zero segments. Such an interleaved arrangement works well, althoughit is not required. The width of the eight segments in the centralregion are all the same (i.e., regions 655 b-655 c; 655 c-655 d; 655d-655 e; 655 e-655 f; 655 f-655 g; 655 g-655 h; 655 h-655 i and 655i-655 j). The two end segments (i.e., regions 654 a-655 a and 655 j-654b) are each twice as wide as the central segments. With thisarrangement, the positive deviation from the reference parabola isoffset by the negative deviations over the face of the reflector. It isworth noting that in the illustrated embodiment, the integral of thesecond derivative deviation over the X-axis is nominally zero.Generally, in order to have a small, well defined flux line at thereceiver it is desirable that the integral of the second derivativedeviation over the X-axis position be nominally zero.

Although the specific curvatures illustrated in FIGS. 7A and 7B workwell, it should be appreciated that the actual geometry of the reflectorsurface may be widely varied to accomplish the same purpose. Thus, forexample, the number of distinct segments and their relative widths canbe widely varied. In the illustrated embodiment, segments that match thecurvature variations of the reference parabola are interspersed withsegments that vary from the reference parabola. In other embodiments,such segments could be eliminated, additional such segments may be addedor they could be replaced by intermediate sections having differentvalues. In the illustrated embodiment, the various segments haveconstant second derivative deviation values. Again, this is notnecessary, and they could be replaced with segments of varying secondderivative deviation values, although in general that may complicate thedesign somewhat. Furthermore, it should be appreciated that thedescribed type of angular and derivative deviation from a referenceparabola may be applied to any type of parabola segment, includingquarter parabola reflectors, half parabola reflectors, near halfparabola reflectors, full parabola reflectors and others.

In the illustrated embodiment, the interfaces between adjacent sectionsare angularly and spatially continuous which is advantageous whenforming the reflector from a single sheet. This is partially due to thefact that it is generally more difficult to reliably form angulardiscontinuities in a reflector sheet. However, as will be described inmore detail below, in alternative embodiments the reflector may beformed from spatially and/or angular discontinuous segments. In stillother embodiments, the described designs can be beneficially combinedwith secondary optics adjacent to the receiver to increase the flux lineintensity and uniformity.

Referring next to FIG. 9, an alternative reflector geometry thatincludes a plurality of distinct reflector segments will be described.In the illustrated embodiment, the collector is formed from a pair ofspaced apart reflector segments, although it should be appreciated thatin other embodiments, more than two reflector segments may be used. Theillustrated collector includes an upper reflector segment 910 and alower reflector segment 920 that both reflect incident radiation to thesame receiver 930. The reflector segments 910 and 920 are independentand each segment may be independently arranged to deviate from anassociated reference parabola in the manner described above with respectto the other embodiments. That is, each reflector segment 910, 920 maybe configured such that its edge regions direct reflected sunlighttowards a central portion of the receiver 930. Thus, sunlight reflectedfrom both the upper edge 910 a and the lower edge 910 b of the upperreflective surface 910 may be directed to a central region of thereceiver 930. Similarly, sunlight reflected from both the upper edge 920a and the lower edge 920 b of the lower reflective surface 920 may bedirected to the central region of the receiver 930. The sunlightreflected from central regions of the upper and lower reflectivesurfaces may be directed to different parts of the receiver 930 in themanner described above.

The reflector segments may be separated by a small gap 940 so that thereflective surfaces are spatially discontinuous. Preferably the gap 940,if present, would be quite small so that little solar radiation is lostthrough the gap. In other embodiments, the reflective surfaces may bearranged to overlap one another. In still other embodiments, thereflective surfaces may generally abut one another or may be radiallyoffset from one another without a forming a gap through which sunlightcan pass. An advantage of allowing a small gap is ease of assembly andalignment, while an advantage of using overlapping or reflector segmentsis that the collector can have slightly higher efficiency due to reducedloses.

The split reflector arrangement has several potential advantages.Initially, for a given collector aperture, the size (i.e., width) ofeach reflective surface is smaller than the width the reflective surfacewould be if it was formed by a single reflective surface. In someembodiments, the smaller width of reflective surface may be morecompatible with low cost, high volume manufacturing than larger widthreflectors. This is particularly noticeable when the sheets of metalused to form the reflector surface are more than a couple meters wide.In one specific example, conventional metal forming equipment used toshape automobile body parts may be readily adapted to produce relativelylarge width reflectors. However, lower cost versions of such equipmentare not generally suited for handling metal sheets having a width ofmore than about 1.5-2 meters. When it is desirable to form largecollectors having wider reflective surface than can be accommodated bysuch equipment, it can be cost effective from a manufacturing standpointto split the reflective surface into a plurality of distinct reflectivesections as described herein. The split reflective surfaces can also beadvantageous from an assembly standpoint in large aperture collectorsystems because the smaller width panels used to form the reflectors maybe easier to handle and align during assembly than very wide panels.

One of the challenges of building concentrating solar collectors is toinsure that the reflective surfaces are properly aligned relative to thereceiver and that the tracking system adequately tracks movements of thesun throughout the day and over the course of changing seasons. Therecan be significant losses in system efficiency if some of the reflectedsunlight does not strike the active portion of the receiver (e.g.,active portions of the photovoltaic cells). In some embodiments it maybe desirable to include secondary optics (e.g. mirrors) on the receiversto direct reflected light that would otherwise miss the active portionsof the receiver back towards the receiver. Such receiver enhancementswill be described with respect to FIGS. 8A and 8B. Referring initiallyto FIG. 8A, receiver 800 includes a base 804 and a photovoltaic cell870. The photovoltaic cell 870 may be an individual cell, a cell in anelongated string of cells or multiple adjacent cells. The receiverfurther includes a pair of longitudinally extending mirrors 810 a and810 b located on opposite sides of the photovoltaic cell string 870. Themirrors 810 a and 810 b cooperate to form secondary optics 810. Themirrors are oriented such that light reflected from the reflector thatstrikes one of the mirrors is directed to the photovoltaic cell 870.

The mirrors may optionally be fabricated from the same material as thereflective trough although this is not a requirement. In the embodimentillustrated in FIG. 8A, the mirrors are flat which tends to helpminimize their fabrication costs. However, in alternative embodiments,the mirrors may be outwardly curved as illustrated in FIG. 8B.

The secondary optics 810 may be used in a variety of manners. In someembodiments, the secondary optics may be used only to provide tolerancesfor alignment and tracking. However, in other embodiments, the secondaryoptics may be designed to provide further concentration. That is,selected portions of the reflector may be designed to intentionallydirect light towards mirrors—which in turn reflect such light towardsthe photovoltaic cells—while other portions of the reflector aredesigned to direct light directly towards the photovoltaic cells.

One particularly useful applications of the mirror type secondary opticsis to direct light from reflective sections that have foci in front ofreceiver. For example, in some applications it may be desirable toorient the reflector surface such that rays reflected from the lowertrough edge (e.g., 654(b) in FIG. 6) strike the upper mirror 810 a andare then reflected a second time towards the photovoltaic cell 870.Similarly rays originating reflected from the upper trough edge (e.g.,654(a) in FIG. 6) may be directed to the lower mirror 810 b.

The actual size and orientation of the mirrors may be widely varied tomeet the needs of any particular circumstance. By way of example, in areceiver designed for use in quarter parabola type collector systems asillustrated in FIG. 2, using secondary optics mirrors having a lengthapproximately the same as the width of the flux line that are orientedat 80° relative to the face of the photovoltaic cells may effectivelyincrease the target area for capturing reflected sunlight by 35%. Ofcourse the appropriate values for the actual length and orientation ofthe secondary optics mirrors will may significantly based on thegeometry of the reflector and the orientation of the receiver.

Using secondary optics of the nature described with reference to FIGS.8A and 8B has several potential advantages. For example, the use ofsecondary optics may permit the photovoltaic cells 870 to be smaller,thereby reducing system cost. The secondary optics also permit the solarconcentration factor to be increased, thereby improving cell efficiency.By way of example, a standard parabolic trough may operate with a 10×concentration factor. Use of the secondary optics may allow the solarconcentration factor to increase to 20×. The effects of the secondaryoptics may also be considered in the design of the reflector surfacegeometry in a manner that allows the photovoltaic cells to beilluminated more uniformly, which tends to lead to increased cellefficiency. As mentioned earlier, the described designs provide a largertarget area (i.e., the combination of the photovoltaic cell and thesecondary optics) which may allows tracking and mechanical tolerances tobe relaxed without requiring a larger cell size.

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. In the foregoing description, for example, there arereferences to a receiver. A receiver can be understood as one or moresolar cells or a structure that includes one or more solar cells. In theforegoing description, there are characterizations of the shape, angleand geometry (e.g., “parabolic,” “normal to incident sunlight”) ofvarious structures (e.g., “reflective surface”). Such characterizationsare not intended to be unduly limiting and contemplate that thedescribed structures may be approximately similar to but may notperfectly satisfy the ideal mathematical criteria for the cited shapes,angles and geometries. For example, a phrase such as “a section of thereflective surface can form a parabolic curve” can also be understood as“a section of the reflective surface forms a shape approximately similarto a parabolic curve,” “a section of the reflective surface at leastsubstantially forms a parabolic curve,” etc. Additionally, in theforegoing descriptions there are references to a point on a curve,reflective surface and/or receiver. Such descriptions can also beunderstood as referring to a small portion, distance and/or interval onthe same.

In many of the reflector geometries described above, edge regions of thereflective surfaces direct incident solar radiation towards a centralregion of the receiver. The targeted central region of the receiver isnot necessarily the midpoint of the receiver, although the midpoint is auseful reference point in some implementations. Thus, for example, insome specific embodiments it may be desirable to direct rays from theupper edge of the receiver towards a specific central region of thereceiver (e.g., toward a position ⅓^(rd) of the flux line width from thebottom of the flux line) while directing rays from the lower edge of thereceiver toward a distinct specific central region of the receiver(e.g., toward a position ⅓^(rd) of the flux line width from the top ofthe flux line). Of course the actual target position for rays reflectedfrom the upper and lower edges of the reflector can be widely variedwithin the scope of the present inventions.

The reflector geometry enhancements discussed above have primarily beendescribed in the context of photovoltaic concentrating solar systems.However, it should be appreciated that the same principles apply to anyconcentrating solar system, regardless of the nature of the receiver.Thus, it should be appreciated that the described improvements areequally applicable to concentrating solar systems that utilize thermalreceivers. Therefore, the present embodiments should be considered asillustrative and not restrictive and the invention is not limited to thedetails given herein, but may be modified within the scope andequivalents of the appended claims.

1. A solar energy collector suitable for use in a solar energycollection system that tracks movements of the sun along at least oneaxis, the collector comprising: a solar receiver; a reflective surfaceincluding a first edge and an opposing second edge, wherein thereflective surface is arranged to direct light to the receiver in anon-imaging manner to form a flux line on the receiver, wherein solarrays reflected from the opposing edges of the reflective surface aredirected towards a central portion of the flux line and solar raysreflected from selected central portions of the reflective surface aredirected closer to edge portions of the flux line than the solar raysreflected from the edges of the reflective surface.
 2. A solar energycollector as recited in claim 1 wherein the reflective surface isangularly and spatially continuous.
 3. A solar energy collector asrecited in claim 1 wherein the collector includes a plurality ofdistinct reflective surfaces arranged to simultaneously direct incidentlight to overlapping portions of the same solar receiver, wherein thereflective surfaces are each individually angularly and spatiallycontinuous, but the reflective surfaces are at least one of angularlyand spatially discontinuous relative to one another.
 4. A solar energycollector as recited in claim 1 wherein the collector has a geometryconfigured to direct incident solar rays reflected from a first centralportion of the reflective surface generally closer to a first edgeportion of the receiver and to direct incident solar rays reflected froma second central portion of the reflective surface generally closer to asecond edge portion of the receiver located opposite the first edgeportion of the receiver.
 5. A solar energy collector as recited in claim1 wherein the reflective surface includes a plurality of reflectivesections, wherein at least some of the reflective sections vary from areference parabola that approximates a cross sectional shape of thereflective surface.
 6. A solar energy collector as recited in claim 5wherein in a first one of the reflective sections, an angular deviationof the reflective surface from the reference parabola variessubstantially linearly such that a second derivative deviation of thereflective surface from the reference parabola is substantiallyconstant.
 7. A solar energy collector as recited in claim 6 whereinmultiple sections of the reflective surface have substantially constantsecond derivative deviations from the reference parabola.
 8. A solarenergy collector as recited in claim 7 wherein the absolute values ofthe second derivative deviations associated with the each of themultiple sections of the reflective surface are approximately equal. 9.A solar energy collector as recited in claim 5 wherein the referenceparabola is selected from the group consisting of: a quarter parabola, ahalf parabola and a near half parabola.
 10. A solar energy collector asrecited in claim 1 wherein the reflective surface includes a pluralityof reflective sections, each of the reflective sections having asubstantially parabolic shape, wherein the directrices of at least someof the reflective sections vary.
 11. A solar energy collection systemcomprising: a solar collector as recited in claim 10; and a trackingsystem configured to direct incident sunlight towards the plurality ofreflective sections such that the incident sunlight is substantiallynormal to a directrix of a reference parabola that approximates a crosssectional shape of the reflective surface.
 12. A solar energy collectoras recited in claim 1 wherein the reflective surface is arranged todirect the sunlight towards the solar receiver using a single reflectionduring operation of the collector.
 13. A solar energy collector asrecited in claim 1 wherein the receiver includes secondary opticssituated adjacent to a flux line that directs reflected sunlight towardsthe solar receiver.
 14. A solar energy collector as recited in claim 13wherein the secondary optics includes two, nominally flat mirrors.
 15. Asolar energy collector as recited in claim 1, wherein: light reflectedfrom the reflective surface onto the solar receiver forms a flux line onthe solar receiver; and the variation in intensity over a middle sectionof the flux line being less than 20%, wherein the middle section of theflux line includes at least 90% of the energy of the flux line.
 16. Asolar energy collector as recited in claim 1 wherein the reflectivesurface is configured to avoid a high intensity focal spot in front ofthe receiver.
 17. A solar energy collector as recited in claim 1wherein: the reflective surface is configured to reflect the incidentsunlight to form a flux line on the solar receiver and includes areflective midpoint between the first and second edge; the reflectivesurface includes a first reflective region that is bordered by the firstedge and the reflective midpoint and a second reflective region that isbordered by the second edge and the reflective midpoint; the first edge,the second edge and the reflective midpoint are adapted to reflect theincident sunlight substantially toward the center of the solar receiverflux line; the first reflective region is adapted to reflect theincident sunlight substantially along a first half of the flux line; andthe second reflective region is adapted to reflect the incident sunlightsubstantially along a second half of the flux line that is substantiallydistinct from the first half of the flux line.
 18. A solar energycollector as recited in claim 1 wherein the receiver includes at leastone string of photovoltaic solar cells.
 19. A solar energy collector asrecited in claim 1 wherein the receiver is oriented to minimize theangle of incidence for light incident on the receiver.
 20. Aphotovoltaic solar energy collector suitable for use in a solar energycollection system that includes the collector, a stand that supports thecollector and a tracking system that causes the collector to trackmovements of the sun along at least one axis, the collector comprising:a solar receiver including at least one photovoltaic solar cell; and areflective surface that is spatially and angularly continuous, thereflective surface including a plurality of reflective sections, a firstedge and an opposing second edge, each of the reflective sectionsincluding a longitudinally extended parabolic curve with a directrixdistinct from the directrix of an adjacent one of the reflectivesections, wherein the directrices of the reflective sections aresubstantially parallel, and wherein the reflective surface is arrangedto direct light to the receiver in a non-imaging manner; and thetracking system is configured such that the incident sunlight issubstantially normal to the directrices.
 21. A solar energy collectorsuitable for use in a solar energy collection system that tracksmovements of the sun along at least one axis, the collector comprising:a solar receiver; and a reflective surface that is spatially andangularly continuous, the reflective surface including a first edge, anopposing second edge and a plurality of curved reflective sections, afirst edge and an opposing second edge, each of the reflective sectionsincluding a longitudinally extended parabolic curve with a directrixdistinct from the directrix of an adjacent one of the reflectivesections, wherein the directrices of the reflective sections aresubstantially parallel, and wherein the reflective surface is arrangedto direct light to the receiver in a non-imaging manner.